Code division multiple access (CDMA) signalling is particularly useful in wireless communications systems, such as cellular telephone systems. Among its advantages, CDMA allows multiple users to simultaneously access a single channel. In a typical CDMA system, a pseudo-noise spreading code (in a direct sequence system a sequence of "chips") is used to biphase modulate an RF carrier. The resulting phase-coded carrier is in turn biphase modulated by a data stream. A second orthogonal code overlays the spreading code which allows a base station to individually identify and communicate with multiple mobile units. The resulting coded CDMA signal is then amplified and transmitted. At the receiver, the CDMA signal is despread and the data extracted by demodulation.
The performance of all wireless communications systems, including CDMA systems, is adversely affected by interference. One source of interference at the base station is caused by the simultaneous receipt of signals from multiple remote (mobile) units, and in particular when those mobile units are broadcasting on the same frequency. Assuming an ideal antenna and signal propagation conditions, and that the base station is receiving signals of substantially the same power from each of the mobile units, the level of interference noise is directly proportional to the number of mobile unit signals received at the base station antenna. The multiple received signals can raise the noise floor or destructively combine to cause fading. This problem is compounded when a mobile unit closer to the base station masks the signals received from mobile units further distant.
Another type of interference which adversely affects wireless communications systems is caused by multipath effects. In this case, the signal broadcast from a given mobile unit will reflect off various objects in the surrounding environment. As a result, multiple reflected signals taking multiple paths of varying path lengths arrive at the receiver. These multipath components (reflections) arrive at the receiver antenna with varying time delays (phase differences), and depending on the corresponding path lengths, may combine to produce fades in signal strength. In the worst case where multipath signals are received one-half wavelength out of phase, a null can occur do to signal cancellation.
By minimizing interference, the strength of a given mobile unit signal received at the base station antenna can be maximized. Consequently, the mobile unit to base station separation and/or the ability to extract data from that signal is improved (i.e. an improved bit-error rate is achieved). A similar result can be achieved if the gain of the receiver and/or its antenna is increased. The most substantial improvements in receiver performance occur if interference minimization is achieved in conjunction with an increase in gain.
The Rake receiver is a standard receiver often used in CDMA base wireless communications systems because of its capability of reducing multipath fading. In one configuration, the Rake receiver receives data from three 120 degree sectors, together providing 360 degree coverage. Each 120 degree sector is covered by two 120 degree antennas with identical views, one antenna feeding the receiver sector port and the other feeding the receiver diversity port. Alternatively, omni-directional antennas may be used to feed a CDMA receiver having only a sector and a diversity port. According to the IS-95 standard, each CDMA receiver is constructed from four Rake receivers, each for resolving one "finger" (i.e. time delayed multipath components from a given mobile unit). In this case, the four strongest signals received from any sector or the diversity antennas are processed by the corresponding four fingers of the receiver and combined to improve data recovery.
It should be noted that in current CDMA receiving systems, the antennas are typically separated by a predetermined number of wavelengths in order to provide spacial diversity. This spacial diversity insures that the incoming multipath components from a given mobile unit transmission are substantially uncorrelated. Two such prior art systems are disclosed in U.S. Pat. No. 5,347,535 to Karasawa et al., entitled "CDMA Communications System," and U.S. Pat. No. 5,280,472 to Gilhousen et al., entitled "CDMA Microcellular Telephone System And Distributed Antenna System Therefor."
If the number of required antennas could be reduced, and/or the need to space antennas by substantial distances could be eliminated, a more compact and less complicated CDMA base station could be built. Further, if in doing so, interference reduction and gain improvement could also be achieved, the receiver operation could simultaneously be improved.
In sum, the need exists for improved apparatus, systems and methods for receiving CDMA signals in a wireless communications system. Such apparatus, systems and methods should reduce fading caused by interference and improve receiver gain. Further, the ability to build a more compact Rake receiver based CDMA receiver system would also be of substantial advantage.